Transport of a passive scalar in wide channels with surface topography: An asymptotic theory

We generalize classical dispersion theory for a passive scalar to derive an asymptotic long-time convection-diffusion equation for a solute suspended in a wide, structured channel and subject to a steady low-Reynolds-number shear flow. Our asymptotic theory relies on a domain perturbation approach for small roughness amplitudes of the channel and holds for general surface shapes expandable as a Fourier series. We determine an anisotropic dispersion tensor, which depends on the characteristic wavelengths and amplitude of the surface structure. For surfaces whose corrugations are tilted with respect to the applied flow direction, we find that dispersion along the principal direction (i.e. the principal eigenvector of the dispersion tensor) is at an angle to the main flow direction and becomes enhanced relative to classical Taylor dispersion. In contrast, dispersion perpendicular to it can decrease compared to the short-time diffusivity of the particles. Furthermore, for an arbitrary surface shape represented in terms of a Fourier decomposition, we find that each Fourier mode contributes at leading order a linearly-independent correction to the classical Taylor dispersion diffusion tensor.

From dynamic self-organization to avalanching instabilities in soft-granular threads

We study the dynamics of threads of monodisperse droplets, including droplet chains and multi-chains, in which the droplets are interconnected by capillary bridges of another immiscible liquid phase. This system represents wet soft-granular matter - a class of granular materials in which the grains are soft and wetted by thin fluid films-with other examples including wet granular hydrogels or foams. In contrast to wet granular matter with rigid grains (e.g., wet sand), studied previously, the deformability of the grains raises the number of available metastable states and facilitates rearrangements which allow for reorganization and self-assembly of the system under external drive, e.g., applied via viscous forces. We use a co-flow configuration to generate a variety of unique low-dimensional regular granular patterns, intermediate between 1D and 2D, ranging from linear chains and chains with periodically occurring folds to multi-chains and segmented structures including chains of finite length. In particular, we observe that the partially folded chains self-organize via limit cycle of displacements and rearrangements occurring at a frequency self-adapted to the rate of build-up of compressive strain in the chain induced by the viscous forces. Upon weakening of the capillary arrest of the droplets, we observe spontaneous fluidization of the quasi-solid structures and avalanches of rearrangements. We identify two types of fluidization-induced instabilities and rationalize them in terms of a competition between advection and propagation. While we use aqueous droplets as the grains we demonstrate that the reported mechanisms of adaptive self-assembly apply to other types of soft granular systems including foams and microgels. We discuss possible application of the reported quasi-1D compartmentalized structures in tissue engineering, bioprinting and materials science.

Rapid Spreading of a Droplet on a Thin Soap Film

We study the spreading of a droplet of surfactant solution on a thin suspended soap film as a function of dynamic surface tension and volume of the droplet. Radial growth of the leading edge (R) shows power-law dependence on time with exponents ranging roughly from 0.1 to 1 for different surface tension differences (Δσ) between the film and the droplet. When the surface tension of the droplet is lower than the surface tension of the film (Δσ > 0), we observe rapid spreading of the droplet with R ≈ t^α, where α (0.4 < α < 1) is highly dependent on Δσ. Balance arguments assuming the spreading process is driven by Marangoni stresses versus inertial stresses yield α = 2/3. When the surface tension difference does not favor spreading (Δσ < 0), spreading still occurs but is slow with 0.1 < α < 0.2. This phenomenon could be used for stretching droplets in 2D and modifying thin suspended films.

Dewetting of Thin Liquid Films Surrounding Air Bubbles in Microchannels

As an air bubble translates in a microchannel, a thin film of liquid is formed on the bounding walls. In a microchannel with a rectangular cross-section, the liquid in the film leaks toward the low-pressure corners of the geometry, which leads to the appearance of local minima in the film thickness in the cross-sectional plane. In such a configuration, theory suggests that the minimum film thickness scales with Ca and Ca^4/3 depending on the distance from the nose of the bubble, where Ca = μU_b/γ is the flow capillary number based on the bubble velocity U_b, liquid viscosity μ, and surface tension γ, and Ca ≪ 1. We show that the film of a partially wetting liquid dewets on the channel wall at the sites of the local minima in the film thickness as it acquires thicknesses around and below 100 nm. Our experiments show that the distance L_w between the nose of the bubble and the initial dewetting location is a function of Ca and surface wettability. For channels of different wettability, L_w always scales proportional to Ca^α, where 1.7 < α < 2 for the range of 10^-5 < Ca < 10^-2. Moreover, L_w increases up to 10 times by enhancing the wettability of the surface at a given Ca. Our present measurements of L_w provide a design constraint on the lengths of bubbles to maintain a liquid wet channel without dry patches on the wall.

Failure mechanisms of air entrainment in drop impact on lubricated surfaces

Lubricated surfaces have recently been introduced and studied due to their potential benefit in various configurations and applications. Combining the techniques of total internal reflection microscopy and reflection interference microscopy, we examine the dynamics of an underlying air film upon drop impact on a lubricated substrate where the thin liquid film is immiscible to the drop. In contrast to drop impact on solid surfaces where even the smallest asperities cause random breakup of the entraining air film, we report two air film failure mechanisms on lubricated surfaces. In particular, using ≈5 μm thick liquid films of high viscosity, which should make the substrate nearly atomically smooth, we show that air film rupture shifts from asperity-driven to a controlled event. At low Weber numbers (We < 2, We = ρ_lU₀²R/σ, U₀ the impact velocity, R the drop radius, and ρ_l the density and σ the surface tension of the droplet) the droplet bounces. At intermediate We (2 < We < 10), the air film fails at the center as the top surface of the drop crashes downward owing to impact-induced capillary waves; the resulting liquid-liquid contact time is found to be independent of We. In contrast, at high We (We > 10), the air film failure occurs much earlier in time at the first inflection point of the air film shape away from the drop center, where the liquid-liquid van der Waals interactions become important. The predictable failure modes of the air film upon drop impact sheds light on droplet deposition in applications such as lubricant-infused self-cleaning surfaces.

Dripping and jetting in microfluidic multiphase flows applied to particle and fiber synthesis

Dripping and jetting regimes in microfluidic multiphase flows have been investigated extensively, and this review summarizes the main observations and physical understandings in this field to date for three common device geometries: coaxial, flow-focusing and T-junction. The format of the presentation allows for simple and direct comparison of the different conditions for drop and jet formation, as well as the relative ease and utility of forming either drops or jets among the three geometries. The emphasis is on the use of drops and jets as templates for microparticle and microfiber syntheses, and a description is given of the more common methods of solidification and strategies for achieving complex multicomponent microparticles and microfibers.

Dripping and jetting in microfluidic multiphase flows applied to particle and fiber synthesis

Dripping and jetting regimes in microfluidic multiphase flows have been investigated extensively, and this review summarizes the main observations and physical understandings in this field to date for three common device geometries: coaxial, flow-focusing and T-junction. The format of the presentation allows for simple and direct comparison of the different conditions for drop and jet formation, as well as the relative ease and utility of forming either drops or jets among the three geometries. The emphasis is on the use of drops and jets as templates for microparticle and microfiber syntheses, and a description is given of the more common methods of solidification and strategies for achieving complex multicomponent microparticles and microfibers.

Michaelis-Menten kinetics in shear flow: Similarity solutions for multi-step reactions

Models for chemical reaction kinetics typically assume well-mixed conditions, in which chemical compositions change in time but are uniform in space. In contrast, many biological and microfluidic systems of interest involve non-uniform flows where gradients in flow velocity dynamically alter the effective reaction volume. Here, we present a theoretical framework for characterizing multi-step reactions that occur when an enzyme or enzymatic substrate is released from a flat solid surface into a linear shear flow. Similarity solutions are developed for situations where the reactions are sufficiently slow compared to a convective time scale, allowing a regular perturbation approach to be employed. For the specific case of Michaelis-Menten reactions, we establish that the transversally averaged concentration of product scales with the distance x downstream as x(5/3). We generalize the analysis to n-step reactions, and we discuss the implications for designing new microfluidic kinetic assays to probe the effect of flow on biochemical processes.

Wetting of flexible fibre arrays

Fibrous media are functional and versatile materials, as demonstrated by their ubiquity both in natural systems such as feathers and adhesive pads and in engineered systems from nanotextured surfaces to textile products, where they offer benefits in filtration, insulation, wetting and colouring. The elasticity and high aspect ratios of the fibres allow deformation under capillary forces, which cause mechanical damage, matting self-assembly or colour changes, with many industrial and ecological consequences. Attempts to understand these systems have mostly focused on the wetting of rigid fibres or on elastocapillary effects in planar geometries and on a fibre brush withdrawn from an infinite bath. Here we consider the frequently encountered case of a liquid drop deposited on a flexible fibre array and show that flexibility, fibre geometry and drop volume are the crucial parameters that are necessary to understand the various observations referred to above. We identify the conditions required for a drop to remain compact with minimal spreading or to cause a pair of elastic fibres to coalesce. We find that there is a critical volume of liquid, and, hence, a critical drop size, above which this coalescence does not occur. We also identify a drop size that maximizes liquid capture. For both wetting and deformation of the substrates, we present rules that are deduced from the geometric and material properties of the fibres and the volume of the drop. These ideas are applicable to a wide range of fibrous materials, as we illustrate with examples for feathers, beetle tarsi, sprays and microfabricated systems.

Steric effects of ions in the charge-related wetting phenomena

Steric effects of ions on the charge-related wetting phenomena are studied. Along with a general treatment, three specific problems in two-dimensional system are considered: a droplet on an electrode, a droplet on a charged surface, and an electrowetting phenomenon on a dielectric. For computation of wetting tension, the electromechanical approach is adopted with the principle of mechanical force balance for each phase. The modified Poisson-Boltzmann equation, which was originally proposed by Bikerman [Philos. Mag. 33, 384 (1942)], is adopted for the analysis of the steric effects. It is found that the steric hindrance reduces significantly both the osmotic pressure and the electrical stress near the triple contact line. This reduction results in a considerable decrease in the wetting tension when the ratio of the capacitance per unit area of the electrical double layer to that of the dielectric layer is small.

Dynamics of wetting: from inertial spreading to viscous imbibition

We report the influence of the nature of boundaries on the dynamics of wetting. We review some work recently published and highlight new experimental observations. Our paper begins with the spreading of drops on substrates and demonstrates how the exponents of the spreading laws are affected either by the surface chemistry or by the droplet shape. We then discuss the imbibition of completely and partially wetting fluids into channels and over microtextured surfaces. Starting with the one-dimensional imbibition of completely wetting liquids in tubes and surface textures, we show that (i) shape variations of channels change the power-law response of the imbibition and (ii) the geometrical parameters of a surface roughness change the spreading behavior. For partially wetting fluids, we observe directionally dependent spreading: polygonal wetted domains can be obtained. We conclude with a tabular summary of our findings, allowing us to draw connections between the different systems investigated, and shed light on open questions that remain to be addressed.

Non-coalescence of oppositely charged drops

Electric fields induce motion in many fluid systems, including polymer melts, surfactant micelles and colloidal suspensions. Likewise, electric fields can be used to move liquid drops. Electrically induced droplet motion manifests itself in processes as diverse as storm cloud formation, commercial ink-jet printing, petroleum and vegetable oil dehydration, electrospray ionization for use in mass spectrometry, electrowetting and lab-on-a-chip manipulations. An important issue in practical applications is the tendency for adjacent drops to coalesce, and oppositely charged drops have long been assumed to experience an attractive force that favours their coalescence. Here we report the existence of a critical field strength above which oppositely charged drops do not coalesce. We observe that appropriately positioned and oppositely charged drops migrate towards one another in an applied electric field; but whereas the drops coalesce as expected at low field strengths, they are repelled from one another after contact at higher field strengths. Qualitatively, the drops appear to 'bounce' off one another. We directly image the transient formation of a meniscus bridge between the bouncing drops, and propose that this temporary bridge is unstable with respect to capillary pressure when it forms in an electric field exceeding a critical strength. The observation of oppositely charged drops bouncing rather than coalescing in strong electric fields should affect our understanding of any process involving charged liquid drops, including de-emulsification, electrospray ionization and atmospheric conduction.

In-fiber semiconductor filament arrays

We report a novel physical phenomenon in which a cylindrical shell undergoing a scaling process evolves into an ordered array of filaments upon reaching a characteristic thickness. We observe that the tendency to breakup is related to the material viscosity in a manner reminiscent of capillary instability. However, unlike the classical breakup of a fluid cylinder into droplets, the structural evolution in our system occurs exclusively in the cross sectional plane while uniformity is maintained in the axial direction. We propose a fluid front instability mechanism to account for the observed phenomena. The fleeting evolution of fluid breakup from a thin film to a filament array is captured in the frozen state by a thermal drawing process which results in extended lengths of solid sub-100 nm filaments encapsulated within a polymer fiber. Hundreds of glassy semiconductor filament arrays are precisely oriented within a polymer fiber matrix making electrical connections trivial. This approach offers unique opportunities for fabrication of nanometer scale devices of unprecedented lengths allowing simplified access and connectivity.

Bridging kinematics and concentration content in a chaotic micromixer

We analyze the mixing properties of the microfluidic herringbone configuration introduced to mix scalar substances in a narrow channel at low Reynolds but large Péclet numbers. Because of the grooves sculpted on the channel floor, substantial transverse motions are superimposed onto the usual longitudinal Poiseuille dispersion along the channel, whose impact on both the mixing rate and mixture content is quantified. We demonstrate the direct link between the flow kinematics and the deformation rate of the mixture's concentration distribution, whose overall shape is also determined.

Influence of substrate conductivity on circulation reversal in evaporating drops

Nonuniform evaporation from sessile droplets induces radial convection within the drop, which produces the well-known "coffee-ring" effect. The evaporation also induces a gradient in temperature and consequently a gradient in surface tension, generating a Marangoni flow. Here we investigate theoretically and experimentally the thermal Marangoni flow and establish criteria to gauge its influence. An asymptotic analysis indicates that the direction of the flow depends on the relative thermal conductivities of the substrate and liquid, k_{R} identical withk_{S}/k_{L}, reversing direction at a critical contact angle over the range 1.45<k_{R}<2. We corroborate the theory experimentally and demonstrate that the Marangoni flow can significantly influence the resulting patterns of particle deposition.

Coalescence of spreading droplets on a wettable substrate

We investigate experimentally and theoretically the coalescence dynamics of two spreading droplets on a highly wettable substrate. Upon contact, surface tension drives a rapid motion perpendicular to the line of centers that joins the drops and lowers the total surface area. We find that the width of the growing meniscus bridge between the two droplets exhibits power-law behavior, growing at early times as t1/2. Moreover, the growth rate is highly sensitive to both the radii and heights of the droplets at contact, scaling as ho3/2/Ro. This size dependence differs significantly from the behavior of freely suspended droplets, in which the coalescence growth rate depends only weakly on the droplet size. We demonstrate that the scaling behavior is consistent with a model in which the growth of the meniscus bridge is governed by the viscously hindered flux from the droplets.

Monodisperse Double Emulsions Generated from a Microcapillary Device

Double emulsions are highly structured fluids consisting of emulsion drops that contain smaller droplets inside. Although double emulsions are potentially of commercial value, traditional fabrication by means of two emulsification steps leads to very ill-controlled structuring. Using a microcapillary device, we fabricated double emulsions that contained a single internal droplet in a core-shell geometry. We show that the droplet size can be quantitatively predicted from the flow profiles of the fluids. The double emulsions were used to generate encapsulation structures by manipulating the properties of the fluid that makes up the shell. The high degree of control afforded by this method and the completely separate fluid streams make this a flexible and promising technique.

Foam drainage on the microscale

The drainage of liquid through a foam involves flow in channels, also called Plateau borders, which generally are long and slender. We model this flow by assuming the flow is unidirectional, the shear is transverse to the flow direction, and the liquid/gas interfaces are mobile and characterized by a Newtonian surface viscosity, which does not depend on the shear rate. Numerical finite difference simulations are performed, and analytical approximations for the velocity fields inside the channels and the films that separate the bubbles are given. We compare the liquid flow rates through interior channels, exterior channels (i.e., channels contacting container walls) and films. We find that when the number of exterior channels is comparable to the number of interior channels, i.e., narrow container geometries, the exterior channels can significantly affect the dynamics of the drainage process. Even for highly mobile interfaces, the films do not significantly contribute to the drainage process, unless the amount of liquid in the films is within a factor of ten of the amount of liquid in the channels.

Foam drainage on the microscale II. Imaging flow through single Plateau borders

The liquid in foam forms an interconnected network, which is composed of Plateau borders, nodes, and films. One of the dominant pathways for foam drainage is flow through Plateau borders, and we use confocal microscopy to obtain experimental results for the flow fields inside individual Plateau borders. For three types of surfactants detailed comparisons are made with a model based upon the influence of surface viscosity at free boundaries between the gas in the bubbles and the liquid in the Plateau borders. The model describes the flows well, and we find good agreement between the surface viscosity predicted by this model and representative values found in the literature. We also give a qualitative description of the flow in the nodes.

Geometrically mediated breakup of drops in microfluidic devices

Microfluidic technology offers capabilities for the precise handling of small fluid volumes dispersed as droplets. To fully exploit this potential requires simultaneous generation of multiple size droplets. We demonstrate two methods for passively breaking larger drops into precisely controlled daughter drops using pressure-driven flow in simple microfluidic configurations: (i) a T junction and (ii) flow past isolated obstacles. We quantify conditions for breakup at a T junction and illustrate sequential breakup at T junctions for making small drops at high dispersed phase volume fractions.

Ordered clusters and dynamical states of particles in a vibrated fluid

Fluid-mediated interactions between particles in a vibrating fluid lead to both long range attraction and short range repulsion. The resulting patterns include hexagonally ordered microcrystallites, time-periodic structures, and chaotic fluctuating patterns with complex dynamics. A model based on streaming flow gives a good quantitative account of the attractive part of the interaction.

Pressure-driven laminar flow in tangential microchannels: an elastomeric microfluidic switch

This paper describes laminar fluid flow through a three-dimensional elastomeric microstructure formed by two microfluidic channels, fabricated in layers that contact one another face-to-face (typically at a 90 degree angle), with the fluid flows in tangential contact. There are two ways to control fluid flow through these tangentially connected microchannels. First, the flow profiles through the crossings are sensitive to the aspect ratio of the channels; the flow can be controlled by applying external pressure and changing this aspect ratio. Second, the flow direction of an individual laminar stream in multiphase laminar flow depends on the lateral position of the stream within the channel; this position can be controlled by injecting additional streams of fluid into the channel. We describe two microfluidic switches based on these two ways for controlling fluid flow through tangential microchannels and present theoretical arguments that explain the observed dependence of the flow profiles on the aspect ratio of the channels.

Dynamic, self-assembled aggregates of magnetized, millimeter-sized objects rotating at the liquid-air interface: macroscopic, two-dimensional classical artificial atoms and molecules

This paper describes self-assembly of millimeter-sized, magnetized disks floating on a liquid-air interface, and rotating under the influence of a rotating external magnetic field. Spinning of the disks results in hydrodynamic repulsion between them, while the rotating magnetic field produces an average confining potential acting on all disks. The interplay between hydrodynamic and magnetic interactions leads to the formation of patterns. Theoretical analysis of hydrodynamic and magnetic forces indicates that the interactions in this system are similar to those acting in systems of finite numbers of particles behaving classically ("classical artificial atoms"). Macroscopic artificial atoms and molecules are described, and the rules governing their morphologies outlined.

Dynamics of coarsening foams: accelerated and self-limiting drainage

The evolution of a foam is determined by drainage flow of the continuous (liquid) phase and coarsening (aging) of the dispersed phase (gas bubbles). Free-drainage experiments with slow- and fast-coarsening gases show markedly different dynamics and elucidate the importance of the coupling of the two effects. Strong coarsening leads to drainage times that are shorter (accelerated drainage) and independent of the initial liquid content (self-limiting drainage). A model incorporating the physics of both drainage and diffusive coarsening shows quantitative agreement with experiment.

Convection, heaping, and cracking in vertically vibrated granular slurries

We address the situation in which vibration is applied to thin layers of granular material with water filling the pore space, but with air above. Beyond a critical drive parameter we observe heap-shaped convecting domains of nontrivial topology, which exhibit cracks as the forcing amplitude is further increased. We summarize these results in a phase diagram, understand the onset of the convecting heaps as a manifestation of the Rayleigh-Taylor instability for fluids, and measure the response of isolated convecting structures.

An accurate von Neumann's law for three-dimensional foams

The diffusive coarsening of 2D soap froths is governed by von Neumann's law. A statistical version of this law for dry 3D foams has long been conjectured. A new derivation, based on a theorem by Minkowski, yields an explicit analytical von Neumann's law in 3D which is in very good agreement with detailed simulations and experiments. The average growth rate of a bubble with F faces is shown to be proportional to F1/2 for large F, in contrast to the conjectured linear dependence. Accounting for foam disorder in the model further improves the agreement with data.

Electroosmotic Flows Created by Surface Defects in Capillary Electrophoresis

We compute the electroosmotic flow in nonuniformly charged planar and cylindrical capillaries for the limit of low-Reynolds-number flows and thin Debye layers. Analytical formulae for the velocity field are provided for the general case of an arbitrary surface inhomogeneity but we also focus on various specific defect geometries. Many important features can be obtained from the simple lubrication approximation. The pressure jump induced by the presence of such surface defects is calculated and the possible occurrence of recirculating flows is discussed, as are effects of the flow perturbations on dispersion in capillary electrophoresis. Copyright 1999 Academic Press.

Resistance to a malignant lymphoma in chickens is mapped to subregion of major histocompatibility (B) complex

A genetic recombinant within the major histocompatibility (B) complex of the chicken has revealed the chromosomal subregion effecting resistance to Marek's disease--a malignant lymphoma induced by a herpesvirus. The recombinant, BF21-G19, occurred spontaneously among the progeny of a male heterozygous for resistant BF21-G21 and susceptible BF19-G19 haplotypes. Exposure to Marek's disease of families segregating for the recombinant showed that this new F-G arrangement conferred a level of resistance equivalent to that of the resistant parental haplotype. Thus, a gene, or genes, within or closely linked to the B-F region of the B complex appears to be responsible for the observed resistance to Marek's disease.

Immune response and disease resistance in chickens. I. Selection for high and low titer to Salmonella pullorum antigen

A selection experiment for high and low anti-Salmonella pullorum antibody titer was carried out over four generations within the B1B1 blood group genotype in chickens. The study was aimed primarily at identifying different response patterns controlled by immune response genes linked to the B system, the major histocompatibility complex of the chicken. Maximal divergence was obtained in the third generation of selection when agglutination titers of 1/320 and 1/80 in B1B1 high and low responders, respectively, were reached. Immune response of S. pullorum was deduced to be controlled by polygenes. The B1B1 population, selected for high immune response to S. pullorum antigen, consistently had greater total mortality as well as greater susceptibility to challenge with Marek's disease virus compared with B1B1 population selected for low response. This, however, is believed to be a consequence of random drift of genes for disease resistance in the relatively small populations and not a direct consequence of selection for high or low S. pullorum titer.

Reliability of Arrhenius Equation in predicting vitamin A stability in multivitamin tablets

Shelflife predictions of vitamin A stability in multivitamin tablets were calculated based on data obtained from the analyses of six multivitamin tablet preparations by the classical Arrhenius treatment. This approach gave excessive variation in the predictions. The same data were treated by a modified Arrhenius method, which reduced the variation. Long-term room temperature data show that incorrect slopes were predicted in three of the six sets of data with the modified approach. The data indicate that erroneous predictions can result when applying the Arrhenius treatment to accelerated data and that this approach should be used with extreme caution in establishing the expiration date of a multivitamin tablet product, especially where the limiting ingredient for expiration dating is a preprocessed raw material and not a single-entity chemical.

Marek's disease: effects of B histocompatibility alloalleles in resistant and susceptible chicken lines

Lines of chickens selected from a common ancestral population for either resistance or susceptibility to Marek's disease developed contrasting frequencies of particular B alloalleles. Comparison of inoculated sibs in backcross-families revealed that the B alloalleles characterizing the two lines accounted for an eightfold difference in tumor incidence. This genetic difference in tumorigenesis associated with the alloalleles of the major histocompatibility complex is probably expressed through the cell-mediated immune system.

Low oncogenic potential of avian endogenous RNA tumor virus infection or expression

Of chickens either spontaneously producing or exogenously infected in ovo with Rous-associated virus, type O (RAV-O), an endogenous virus of the chicken, only 1 died with lymphoid leukosis (LL), the most common neoplasm associated with the leukosis-sarcoma virus group. Because the chickens were not kept in strict isolation, it could not be assumed that the one LL was induced by RAV-O. In contrast, RAV-1-infected chickens from the same lines had a high incidence of LL and other neoplasms. Over 800 chickens of several inbred lines were maintained in plastic isolators free of exogenous avian leukosis-sarcoma virus infection for from 500 to nearly 1,000 days of age. No LL was observed, even though some lines are known to produce RAV-O spontaneously or to express inherited gs antigen. Three neoplasms of unknown etiology were observed, but none generally associated with leukosis virus infection. We concluded that avian endogenous virus expression had little, if any, oncogenic potential, and that exogenous avian leukosis viruses were responsible for most naturally occurring neoplasms.